1. Field of the Invention
This invention pertains to analog signal processing and more specifically to a circuit for conversion of a voltage to a differential current.
2. Description of the Related Art
Differential converters are well known; an example of such a converter using a standard differential pair of transistors Q1 and Q2 is shown in FIG. 1. An input signal .DELTA.V is applied to the base of transistor Q1. The base of the second transistor Q2 is connected to AC ground. (DC ground is an absolute signal reference at zero volts potential, usually referred to as "ground" or "earth" in the literature. AC ground is a signal reference that may be at a potential other than zero volts. In practice, AC ground is a DC voltage source.) The emitters of transistor Q1 and Q2 are connected to a current sink I.sub.S which sinks a current of 2I.sub.0 where I.sub.0 is the emitter current of each transistors Q1, Q2.
With this circuit it can be shown that I.sub.1 -I.sub.2 =2I.sub.0 (e.sup..DELTA.V/2V.sbsp.T -e.sup.-.DELTA.V/2V.sbsp.T)/(e.sup..DELTA.V/2V.sbsp.T +e.sup.-.DELTA.V/2V.sbsp.T). In this case, V.sub.T is the transistor thermal voltage of each of transistors Q1 and Q2 where they are identical transistors. V.sub.T =kT/q where k is Boltzman's constant, T is temperature in .degree.K., and q is electronic charge. At room temperature, V.sub.T is about 26 mV.
Therefore, it follows that the small signal gain is equal to I.sub.0 /V.sub.T and the maximum output current is equal to .+-.2I.sub.0. Hence undesirably, both the small signal gain and the maximum output current are limited.
It is to be appreciated that there are two output currents I.sub.1 and I.sub.2 ; hence this is referred to as a differential pair since the output signal is the difference I.sub.1 -I.sub.2 between the currents which is a function of the voltage .DELTA.V of the input signal.
As shown above, this function is not truly linear but is approximately linear. It is understood that the input signal .DELTA.V, may be a sinusoidal signal as depicted, for instance an RF (radio frequency) signal.
FIG. 2 shows an improved converter (transconductance circuit) compared to that of FIG. 1. The circuit of FIG. 2 is disclosed in the publication "Motorola's Mosaic V Silicon Bipolar RF Building Blocks Fill Gaps in High Performance, Low Power Wireless Chips", by Durec et al., Wireless Symposium, 1996; see also U.S. Pat. No. 5,497,123 issued Mar. 5, 1996 to main and Durec. Present FIG. 2 is a redrawn version of FIG. 5 from that publication. The circuit of FIG. 2 includes a common base transistor Q4 and current mirror transistors Q5 and Q6 with transistor Q5 connected to function as a diode. When current flows through the input (which is a low impedance node formed at the junction of the emitter of Q4 and the collector base of Q5) the current adds to the quiescent current passing through the current mirror transistors Q5, Q6 and thus raising the input voltage. Resistors R1 and R2 may be used to improve the linearity of the circuit. As the input voltage rises, the current flowing through the common base transistor Q4 decreases. A signal flowing through the input of this circuit appears as a difference between two output currents I1, I2. Thus the circuit converts a single-ended input signal .DELTA.V provided at the input terminal into a differential signal I1-I2. This publication also discloses use of this transconductance circuit in combination with a switch stage, to serve as a frequency mixer.
The FIG. 2 circuit, compared to that of FIG. 1, while being more linear has an inferior noise figure performance and an inferior gain. In this sense it is not a satisfactory solution to convert to current conversion.
Therefore there is still a need for a voltage to current converter providing a differential output current which is a linear function of the input voltage and having low noise and high gain.